The following relates to injection molding machines specifically adapted for plasticizing materials such as thermoplastic materials and the like, although it is adaptable to other materials capable of being formed in a like manner.
In the injection molding machine art, most machines are of the rotatable and reciprocating screw type. It has generally replaced the conventional "plunger" type of machine which consisted of a reciprocal plunger forcing plastic over a fixed torpedo near the outlet portion of the machine which material was injected into a mold.
A limitation of the reciprocating screw type machine is obtaining a mottled or marbleizing effect on injection molded items. The reciprocating screw is so efficient that when colorant is added to the plastic pellets, the resultant effect is a single homogeneous colored plastic article. Attempts to produce marbleizing on the reciprocating screw without major modifications thereto have not been successful. In fact, it is common today to still use the old plunger type machines to produce marbleized items. This results in a substantially slower cycle time and inefficient utilization of the more modern and highly efficient reciprocating screws.
A separate problem associated with conventional reciprocating screw type injection molding machines is that the cycle is substantially fast. The screw moves back into the cylinder, subjecting the plastic pellets to plasticization over a shorter length of the screw, thereby not as effectively melting the plastic. In some instances, the screw rotational speed must be reduced and the machine cycle must be substantially increased to insure that the plastic will be thoroughly plasticized before injection.
It would be desirable then to provide for a reciprocating screw type injection molding machine means to produce marbleized parts. It further would be desirable to decrease the cycle time for a reciprocating screw injection molding machine.